As noted by sometime SV-POW!sketeer Darren Naish over on Tetrapod Zoology, frogs have stupidly weird skeletons — surely the most derived of any tetrapod, despite their lowly, early diverging “amphibian” status. Rather than describe all the oddities myself, I’ll just quote Darren’s article:

Anurans have (at most) nine presacral vertebrae, and some have as few as five; ribs are either highly reduced or absent; the radius and ulna are fused (forming the radioulna); the bones of the pectoral girdle are highly reduced and complimented by an assortment of new weird bits; the pelvis consists of a rod-like central unit (the urostyle) surrounded by two super-long, shaft-like ilia; and in their (generally) elongate hindlimbs, the tibia and fibula are fused (forming the tibiofibula) while the ankle bones are elongated to form a long ‘extra’ limb segment.

That’s a pretty astonishing list; and, sure enough, frog skeletons weird me out every time I see them. (Of all the dead animals I’d like to get hold of in decent condition, to extract the bones from, frogs miss the top of the list only to bats, crocs and turtles. And maybe raptors.)

This particular species of frog has another skeletal oddity that caught my eye:

As you can see, the humerus is perforated: there is a distinct foramen running down it just behind the anterior edge. Is the anterior bar a partially detached deltopectoral crest? Or is it a completely novel ossification that has become partially fused to the humerus?

For what it’s worth, this feature doesn’t seem to be consistently present in goliath frogs. A bit of googling shows that it’s present in this skeleton, but not in this one from Bone Clones. The humerus of the latter does have a distinctly protruding deltopectoral crest, but it lacks the perforation. So I guess that is evidence against the Novel Ossification hypothesis.

Does anyone know more about this odd feature? Does it develop through ontogeny? Is it found in other frogs? What is its mechanical significance?

Hyperossified frogs have usually thick bones and sculptured, fused skulls. A large frog isn’t essentially hyperossified, and not all hyperossified species are very large. I have a great fascination with these species, as they could give us a glimpse back to paleozoic forms. Hyperossification starts quite early in their development, and it is considered a peramorphic feature. This, together with other weird features like non-pedicellate teeth and reapearance of bones that do not have the time to ossify in most species might suggest atavisms to paleozoic amphibians. Most hyperossified species are either fierce carnivores or burrowers, and in the first case at least they resemble hyperossified temnospondyls.
Why most modern amphibians, even strongly terrestrial ones, are flimsy, cheep to make versions of their ancestors is a mystery to me. Why they lose their strong bones for a nearly cartilaginous frame like they evolved to be eaten or smashed? Many have toxins to protect them, but not all, and we are not sure if the older, hyperossified forms had toxins as well. Very rarely you will find species with real skulls or bony plates. The same happened to fish, of which most teleosts have just nominal scales and spine-like bones. Why? Was the availability of calcium greater back then? Or they got under competition from other vertebrate lineages?
I have also a pet Cranwell’s horned frog, Ceratophrys cranwelli, which is a little hyperossified monster that can attack everything. It is indeed very different from other, more typical frogs both in its lifestyle and in its mode of eating. It also has osteoderms overlaying its vertebrae for extra protection, Cacops also had those, and also had a similar body shape. The topic becomes even more interesting if it is true that cacopids are most closely related to lissamphibians. Could both have a similar ecology? Could these osteoderms be atavisms, or just neomorphs? From what I know, no such study has been undertaken. I also plan to get a Pyxicephalus adspersus to compare the two. Pyxicephalus has not the osteoderms though. It is just a fortified ranid. But really fortified.

You’re right, Bolko, I mistitled this post — as Marc Evans pointed out in the tweet I quoted above. In fact I titled it after a poster he’d presented at one of my first SVPCAs, and which tickled my fancy.

Bolko – I think you hit the nail on the head when you said they were “cheap to make”. Modern amphibians have largely gone down the path of high-r-selection – grow fast, mature early, breed profligately. This is why they’re small (Conraua and even Andrias are puny by Paleozoic standards) and lightly built.

Building a massive skeleton is a big investment that takes resources away from growth and reproduction. Besides, what good would it do? Most of the predators that frogs face outsize them by an order of magnitude or more; a heavy skeleton would not deter a snake, perciform, or heron that could swallow the frog whole, or the powerful jaws of turtles, crocs, or carnivorans.

Mike – I seldom comment but I’m a regular reader. You guys are awesome. Keep up the good work!